Staphylococci are gram-positive bacteria which normally inhabit and colonize the skin and mucus membranes of humans. If the skin or mucus membrane becomes damaged during surgery or other trauma, the Staphylococci may gain access to internal tissues causing infection to develop. If the Staphylococci proliferate locally or enter the lymphatic or blood system, serious infectious complications such as those associated with Staphylococcal bacteremia may result. These complications include septic shock, endocarditis, arthritis, osteomyelitis, pneumonia, and abscesses in various organs.
Staphylococci include both coagulase-positive organisms that produce a free coagulase and coagulase-negative organisms that do not produce this free coagulase. Staphylococcus aureus is the most common coagulase-positive form of Staphylococci. S. aureus generally causes infection at a local site, either extravascular or intravascular, which ultimately may result in bacteremia. S. aureus is also a leading cause of acute osteomyelitis, and causes Staphylococcal pneumonia infections. Additionally, S. aureus is responsible for approximately 1-9% of the cases of bacterial meningitis and 10-15% of brain abscesses.
There are at least thirty-one known species of coagulase-negative Staphylococci, including S. epidermidis, S. saprophyticus, S. hominis, S. warneri, S. haemolyticus, S. saprophiticus, S. cohnii, S. xylosus, S. simulans, and S. capitis. S. epidermidis is the most frequent infection-causing agent associated with intravenous access devices, and the most frequent isolate in primary nosocomial bacteremias. S. epidermidis is also associated with prosthetic valve endocarditis.
Staphylococcus is also a common source of bacterial infection in animals. For instance, Staphylococcal mastitis is a common problem in ruminants such as cattle, sheep, and goats. The disease is generally treated with antibiotics to reduce the infection but the treatment is a costly procedure and still results in a loss of milk production. The most effective vaccines identified to date are live, intact S. aureus vaccines administered subcutaneously. The administration of live vaccines, however, is associated with the risk of infection. For that reason, many researchers have attempted to produce killed S. aureus vaccines and/or to isolate capsular polysaccharides or cell wall components which will induce immunity to S. aureus. 
Carrier compounds are sometimes used in vaccines in order to enhance the immune response to the antigen. For example, the carrier in some vaccines is useful for stimulating T cell help in response to the antigenic moiety. Antigenic moieties that are naturally occurring or fragments of naturally occurring substances are sometime less amenable and less facilely manipulated and/or conjugated to other moieties such as for example carrier compounds, and this can reduce the therapeutic impact of such vaccines.